A first set of test structures for a gallium nitride (GaN) transistor that includes N field plates is disclosed, where N is an integer and X is an integer between 0 and N inclusive. A test structure TSX of the first set of test structures includes a GaN substrate, a dielectric material overlying the GaN substrate, a respective source contact abutting the GaN substrate and a respective drain contact abutting the GaN substrate. The test structure TSX also includes a respective gate overlying the substrate and lying between the respective source contact and the respective drain contact and X respective field plates corresponding to X of the N field plates of the GaN transistor, the X respective field plates including field plates that are nearest to the GaN substrate.